Can I modify a rigify rig to add a head after generating it?
https://docs.blender.org/manual/en/2.81/addons/rigging/rigify.html
From the manual:
Nondisruptive re-rig
If the generated rig doesn’t fit all the features you need or, for example, you decide to add something more to your character (like a sixth arm or a tail), you can re-generate your rig without losing your previously generated features and your animation data.